In recent years, wireless communication using radio frequency (RF) and light is widely performed. For example, a cellular phone using a magnetic wave in a 900 to 1900 MHz band is indispensable as an anytime-and-anywhere communicable wireless mobile communication tool. Further, as communication means between personal computers or between a personal computer and a printer, wireless communication using a radio wave in a 2.45 GHz band in conformity with wireless LAN (IEEE 802.11b or IEEE 802.11a) or Bluetooth standard, is realized and widely used in offices and houses. Further, a Suica (registered trademark) card, adopted by East Japan Railway Company in 2001, realizes non-contact reading and writing of ticket at a station ticket gate.
The above tools are about palm-sized products, however, coin-sized or smaller wireless communication tools, so-called radio tags are used for personal identification (ID) purpose and the like. For example, an anti-theft automobile key is realized by using a small radio tag of about 1 cm embedded in the key thereby enabling an authentication operation upon turning-on of the key. In addition, in a case where cable communication cannot be performed without difficulty between e.g. a mountain top place and a point at the ground, optical communication is employed for high-speed communication of images obtained by a camera.
Further, a value-added network incorporating a large number of sensors has been actively developed. For example, proposed is a system as a precaution against earthquake, constructed with an acceleration sensor and a strain sensor embedded into a steel frame of a building structure for measuring the degree of fatigue of the steel frame or concrete.
Further, recently, a wireless sensing network is positively proposed.
Nikkei Electronics issued Jul. 15, 2002 introduces in pp 99 to 129, several sensing network systems. For example, a large number of sensors are distributed in a forest, and the amount of generated oxygen, the amount of absorbed CO2, the temperature, humidity of the forest and the like are grasped in detail, for the purpose of early detection of bush fire or CO2 emission trading.
Further, an imaging network by optical sensing may be constructed. Among various types of lenses for an imaging system U.S. Pat. No. 5,004,328 discloses a spherical lens 1 including an aperture 2 in its central portion as shown in FIG. 11. It is known that the aperture improves image formation characteristic of the imaging system.
In a case where a wireless sensing network is constructed by arranging a large number of sensing devices (hereinbelow, each referred to as an “Mote”), to sense a slight environmental change in a wide range, the size of the Mote must be small. Further, in wireless communication, to perform communication with a place as far as possible, the size of antenna must be as large as possible.
Accordingly, in a case where a wireless sensing network by wireless communication is constructed for sensing a slight environmental change in a wider range, it is necessary to reduce the size of the Mote and increase the size of the antenna for wireless communication.
However, if the antenna for wireless communication is provided in the Mote, the size of the Mote increases, which disturbs construction of wireless sensing network for excellent sensitivity to environmental change. Further, as the structure of the Mote is complicated, a manufacturing process of the Mote becomes complicated.
Further, upon construction of a sensing network system with distributed devices having sensing and communication functions (hereinbelow, each referred to as a “sensing Mote”), if each sensing Mote has a high performance capability, the cost is increased by arranging a large number of sensing Motes.
Further, in a case where the sensing Mote is reduced in size for sensing a slight environmental change, the sensing function installed in the small sensing Mote is limited.